In the manufacture of fiber lightguides for use in communications, there are several different techniques for producing an optical fiber, for example. One such technique comprises directing a constantly moving stream of gas phase reactants and oxygen through a glass substrate tube. The tube may be silicon dioxide (Si O.sub.2), for example, and the reactant, silicon tetrachloride (Si Cl.sub.4). The oxygen stream will also carry dopants to produce the appropriate or desired index of refraction in the finished optical fiber. The substrate and the glass are heated to a reaction temperature within a moving hot zone traversing the outside of the tube, and the consequent reaction produces Si O.sub.2 and dopants fused into a continuous layer on the inner wall of the tube within the hot zone.
Generally, the heating is accomplished by one or more gas nozzles which directs jets of burning gases onto the surface of the tube. See, for example, U.S. Pat. No. 3,982,916 issued Sept. 28, 1976 to S. E. Miller. A single nozzle, or preferably a plurality of nozzles surrounding the tube produce a temperature profile on the surface of the tube, and, hence, inside the tube, with a peak value sufficient to accomplish the desired reaction and deposition. As can be expected, the nozzles become extremely hot in operation, and, as a consequence, some of the material of the nozzles is oxidized and carried with the flame to the surface of the tube. Such a flame is commonly referred to as a "dirty" flame, and is to be avoided.
In those installations in which nozzles are directed toward the tube, there is a great deal of wasted heat and inefficiency, requiring greater amounts of combustible gases to achieve the desired temperatures. In addition, the flame is not confined, making a precise control of the temperature profile difficult. Efforts to increase efficiency have included mounting the nozzles in a ring-shaped housing which surrounds the tube and helps confine the heat to a zone or area of the tube. Such an arrangement is used in a process for drawing a glass tube into a rod and is shown, for example, in U.S. Pat. No. 3,652,248 of Loxley et al. Efforts to control the heating efficiency and temperature profile are successful for one set of parameters, among which are gas composition, gas velocity, and proximity of the nozzles to the tube. However, if any of these parameters are changed for an arrangement such as shown in the Loxley et al patent, for example, it is necessary to use a totally different housing structure. Furthermore, if attempts to vary the temperature profile include increasing the gas velocity or varying the constituent elements of the gases, damage to the tube, such as erosion of the outer surface, may result. Also, relatively high velocities cause extremely turbulent gas flows which are difficult to control.